1. Field of the Invention
The present invention relates to a belt conveying apparatus wherein a convey belt is wound around and supported by at least two rollers, and more particularly, it relates to a belt deflection adjustment technique in a belt conveying apparatus wherein a convey belt is apt to deflect in a direction perpendicular to a conveying direction at a roller supporting mechanism.
2. Related Background Art
Conventionally, for example, in automatic original (document) feeding apparatuses, an original has been fed to a predetermined position on a platen glass by using a convey belt, and, in image forming apparatuses, a convey belt has been used with a sorter for discharging sheets on which an image was formed in order and for arranging the sheets properly.
Generally, as shown in FIGS. 10A to 10C, the convey belt 1 has a core (woven cloth or web) 12 comprising peripheral lengthwise threads (weft threads) 10 and widthwise threads (warp threads) 11. A front (outer) surface of the core 12 is coated by a surface member 13 made of rubber or synthetic resin, and a back (inner) surface of the core is also coated by a surface member 15. The core 12 is formed by weaving the warps 11 and the wafts 12 alternately in an endless fashion.
When the convey belt 1 incorporating the core body therein and wound around two or more rollers is rotated, there arises a phenomenon that the belt is defected leftwardly or rightwardly. This phenomenon is caused by the defect of a mechanism portion such as unevenness of roller shaft interval and/or unevenness of roller shaft diameter, or by the defect of the belt itself such as difference between left and right peripheral-lengths of the belt and/or difference of twist of lengthwise threads (wefts) in the belt. Further, regarding the deflection of the belt, there is a case where the belt is deflected toward either side regardless of an advance direction of the belt (this case is referred to as directionality "exist") and a case where the direction of deflection of the belt is varied as the advance direction of the belt is changed (this case is referred to as directionality "no").
FIG. 11 shows a relation between the cause of the deflection of the convey belt and the directionality. In FIG. 11, when the cause of the deflection depends upon the unevenness of the shaft interval, two cases A and B can be considered.
As shown in FIG. 12, A is a case where, although axes of rollers 2, 3 are lying on the same plane, these axes are not parallel with each other; whereas, as shown in FIG. 13, B is a case where the axes of the rollers 2, 3 are not lying on the same plane. Regarding the twist direction of the thread in FIG. 11, the cause of the deflection is arisen by the twist direction of the wefts (peripheral lengthwise thread) 10 of the belt. That is, when the S-twisted thread wherein the thread is twisted rightwardly (FIG. 2A) is used, the belt is likely to be deflected leftwardly with respect to the advance direction of the belt, and, when the Z-twisted thread wherein the thread is twisted leftwardly (FIG. 2B) is used, the belt is likely to be deflected rightwardly.
In order to prevent such deflection of the belt, there have been proposed various techniques. For example, flanges were provided on both ends of each roller at the mechanism portion, or the S-twist threads and the Z-twisted threads were alternately arranged in the belt to cancel the left and right deflection forces, or, as disclosed in the Japanese Utility Model Laid-Open No. 63-162742, a deflection stopper was added to the belt.
However, if the mechanism includes the cause of the deflection of the belt, even when the belt comprising the S-twisted threads and Z-twisted threads arranged alternately is used, the belt will be deflected. If the deflection force of the belt becomes too great, the belt will often ride over the flange on the roller and the deflection stopper to deflect outwardly, and, thus, in some cases, only by using the S-twisted threads and the Z-twisted threads, the problem of the belt deflection cannot be solved. Further, although the technique that the S-twisted threads and the Z-twisted threads are used alternately permits the production of a belt which is formed by winding the threads spirally around a mold, in a seamless weaving method wherein the threads are woven in a grid pattern, it is substantially impossible to weave two kinds of threads having different weave direction alternately.
Thus, conventionally, the belt has been put to practical use by adopting the following techniques:
(1) Increasing the thickness of a belt; PA1 (2) Increasing the resiliency of a belt by coating hard material on the belt; or PA1 (3) Providing a belt deflection preventing device.
However, with these techniques, the following problems arise.
That is to say, in order to suppress the deflection of the belt by means of the flanges secured to the roller ends, it is necessary to increase the rigidity of the belt so as to prevent the bending of the belt edge due to the reaction force of the flanges. Although the rigidity of the belt is increased by increasing the thickness of the belt or by coating the hard material on the belt, if the rigidity of the belt is great, the belt cannot follow an outer periphery of a small diameter roller adequately, and, therefore, the minimum diameter of the roller to be used with the belt becomes greater, thus limiting a range of the usable diameter of the roller. Further, in a thicker belt, there arises a difference in length between an outer peripheral surface of the belt wound around the roller and an inner peripheral surface of the same belt wound around the same roller, thereby creating difference of peripheral length, which results in difference of peripheral speed of the belt. This phenomenon has a bad influence upon not only the smooth sheet conveyance but also the service life of the belt considerably.
Further, when the exclusive belt deflection preventing device is provided, the conveying apparatus becomes more expensive and more complicated, and the smooth conveyance of the sheet is deteriorated. Furthermore, in the above techniques, since the deflection of the belt toward the either side is forcibly suppressed, if the belt having the strong deflection force is contacted with any device, the great resistance force will be generated at the contact area. Since such resistance force must be compensated for by increasing a driving force for the belt, consumption of electric power is increased. In addition, since the edge of the belt is rubbed when the belt is contacted with the belt deflection preventing device, the edge of the belt is damaged.